The present invention relates to jet printing devices and, more particularly, to apparatus for controlling accurately the deposit of ink drops on a print receiving medium to produce collectively an image of high resolution.
Jet drop recorders, such as that shown in U.S. Pat. No. 3,701,998, issued Oct. 31, 1972, to Mathis, have included one or more rows of orifices which receive electrically conductive recording fluid, such as water base ink, from a pressurized fluid manifold and eject the fluid as parallel fluid filaments. Mechanical stimulation is applied to the structure or is fluid-coupled to the orifices, causing each of the filaments to disintegrate into a jet drop stream.
Graphic reproduction in recorders of this type is accomplished by selectively charging some of the drops in each of the streams. The drops then pass through an electric field which deflects the charged drops such that they strike a drop catching device. The uncharged drops, however, pass unaffected through the deflection field and are deposited on a moving web of paper or other material. The uncharged drops collectively form a print image on the web.
A problem with jet printers has been attaining sufficient image resolution. Since a discrete number of drops from the printed images, it is clear that an increase in the number of drops deposited per unit area of print medium, and a corresponding increase in data handling capability, permit improved image definition. If, however, each jet is used in a binary manner to deposit drops selectively at a single associated position on the print receiving medium, the number of drops per unit width and, therefore, the resolution of an image in the direction transverse to the print web, are limited by the minimum dimensions required between orifices. U.S. Pat. No. 4,010,477, issued Mar. 1, 1977, to Frey discloses a printer in which the effective density across the print receiving medium is increased by orienting rows of jets along angularly positioned placement lines. The Mathis '998 device increased the number of drops across the width of the medium by using multiple rows of jet drop streams which interlace. The printer disclosed in "Ink Jet Head," by Krause, IBM Technical Disclosure Bulletin, Vol. 19, No. 8, January 1977, pp. 3216 and 3217, combines these approaches by providing two interlaced rows of drop streams which are positioned obliquely with respect to the direction of web movement.
It will be appreciated, however, that increased resolution may be obtained by constructing the printer such that drops from each jet can be deflected selectively to any of a number of positions on the print receiving medium. U.S. Pat. No. 3,739,395, issued Jun. 12, 1973, to King discloses a printer in which uncharged drops are caught while the charged drops from each orifice are deflected by two sets of deflection electrodes to a plurality of discrete positions on the moving web. Deflection of the drops is either perpendicular or parallel to the direction of web movement, or both, covering either a one line matrix or a multiple line matrix across the web. The minimum distance between jet orifices is somewhat greater in the King device than in previously mentioned devices, however, since deflection electrodes must be positioned on all sides of each orifice.
U.S. Pat. No. 3,972,052, issued Jul. 27, 1976, to Atumi et al discloses an ink jet printing device in which a single jet scans a plurality of print lines in succession under control of two pairs of deflection electrodes. The electrodes provide parallel deflection fields through which the ink drops pass in succession, with identical ramp deflection voltages being applied to the deflection electrodes. The deflection voltage applied to the second pair of deflection electrodes is delayed with respect to the deflection voltage applied to the first pair so that the same linearly increasing field is experienced by each drop between both pairs of electrodes.
The difficulties encountered with structures in which deflection electrodes are positioned intermediate adjacent jet drop streams have been eliminated by providing a print head which generates a plurality of jet drop streams positioned along a row which is inclined with respect to the direction of web movement, and in which deflection of the drops in the streams to various print positions is accomplished by a deflection field or fields perpendicular to the row or rows of streams. Such arrangements are shown in U.S. Pat. No. 4,085,409, issued Apr. 18, 1978, to Paranjpe and U.S. Pat. No. 4,122,458, issued Oct. 24, 1978, to Paranjpe. In the Paranjpe '409 device, drops are selectively charged to differing print charge levels such that the degree of deflection which they experience as they pass through the deflection field varies correspondingly. In the Paranjpe '458 patent, a printer is disclosed in which the trajectories of the drops are controlled by varying, non-cyclical electric deflection fields. The fields are altered in dependence upon the amount of deflection dictated by the print controlled data.
It can be seen, therefore, that in the Paranjpe '409 and Paranjpe '458 patents, the printers require either charging of drops to multiple discrete levels or non-cyclical high speed of the fluctuation deflection field. Large swings in field intensity are not easily accomplished in short periods of time due to the capacitive nature of the deflection electrodes. Additionally, it will be appreciated that providing a plurality of discrete deflection electrodes for each jet drop stream may result in substantial cross talk between the deflection field controlling adjacent jets. High speed fluctuation in charge potential levels, on the other hand, where many such levels are used, as in the Paranjpe '409 device, is complex and requires substantial control circuitry.
It is seen, therefore, that there is a need for a simple printer capable of high speed printing with a large number of ink drops so as to increase image definition.